1. Field of the Invention
The present invention relates to a brushless motor in which stator coils are connected together and are connected to output wires by use of a bus bar.
2. Description of the Related Art
When a motor is used for drive of an electric power tool or the like, a relatively large current flows through the coils of the motor. Therefore, heat generated as a result of the current flowing through the coils must be removed by means of cooling. If the generated heat is not removed, the current decreases because of an increase in the resistance of the coils, with a resultant decrease in output torque of the motor. A conventional cooling apparatus for a motor has a structure as shown in FIG. 11 (see Japanese Patent No. 2882594). A plurality of radial and axial through holes are formed in a motor casing and an end cap, respectively. A stator, composed of a magnetic-pole core and coils wound around the magnetic-pole core, and switching elements constituting a drive circuit are attached to an inner wall surface of the motor casing. A rotor is fixedly provided on a rotation shaft, and a blower fan is fixedly provided adjacent to the rotor. A small clearance which serves as a cooling air passage is formed between the rotor and the stator magnet-pole core. When electricity is supplied to the stator coils via the drive circuit and the rotor rotates, the coils, the drive circuit, etc., generate heat. However, the blower fan rotates together with the rotor, and as a result of rotation of the blower fan, cooling air entering from the through holes flows through the cooling air passage to thereby cool the coils, the drive circuit, etc.
However, the cooling air merely passes through the clearance between the rotor and the stator, and does not flow along the surfaces of the coils, from which heat is generated. Therefore, heat generated from the coils cannot be directly removed, and therefore a sufficient cooling effect cannot be attained.
Meanwhile, conventionally, wire harnesses or a bus bar is used for connecting wires from an external power source to coils. However, in ether case, the wiring hinders cooling of the motor.
FIG. 12 is a view of a stator of a brushless motor as viewed from a side from which coils are extended, and shows an example in which wire harnesses are used. In order to form a Y connection, one ends of U-phase, V-phase, and W-phase coils of the brushless motor are connected together by use of a Y-connection insulating sleeve. The other ends of these coils are connected to three-phase output wire harnesses, which serve as lead wires, via respective insulating sleeves. The three wire harnesses for three-phase output are routed on the end surfaces of the stator coils as shown in FIG. 12 so as to extend to the outside of the motor housing from a single location. Therefore, the length and size of the motor increase, and due to complexity of the wire connection, the numbers of steps and components increase, with a resultant increase in cost. Further, the routed portions of the wire harnesses and the sleeves for connecting the coils and the wire harnesses close the clearances between the coils wound around the stator core, and therefore cooling air passages cannot be formed there.
FIG. 13 is a view showing a method in which coils are connected by use of a bus bar (see Japanese Patent Application Laid-Open (kokai) No. 2005-328661. As shown in FIG. 13, coils are wound around a stator magnetic-pole core. The bus bar is composed of wiring bar groups for connecting a large number of coil segments which constitute the coils, and an insulating member which supports the wiring bar groups at respective spatial positions while electrically insulating them. The wiring bar groups include three output wires projecting from the bus bar. Through use of such a bus bar, the coils can be connected together and be connected to the output wires. However, as in the above-described case where wire harnesses are used, the clearances between the coils wound around the stator magnetic-pole core are closed by the bus bar, and therefore cooling air passages cannot be formed there.